It is a convenient practice to physically separate a digital computer from the monitor and keyboard by which the computer is monitored and controlled. The monitor and keyboard may be in the form of a single housed unit, such as a terminal, or the keyboard and monitor may be separate components coupled to the computer by cables.
Also as a matter of convenience, there are separate electrical connectors on these computers for receiving mating monitor and keyboard plugs, one plug being connected to a monitor cable from a monitor, and the other plug being connected to a keyboard cable from a keyboard. Normally, these cables are on the order of four to eight feet in length, enabling some, but limited, separation of a monitor and keyboard from a computer. This typically requires that the computer, monitor, and keyboard be generally located together.
It is to be appreciated that there are situations, in fact, many, where it is desirable to separate the computer from the monitor and keyboard. This may be by virtue of space limitations or because of environmental considerations, the latter sometimes including an inhospitable environment for the delicate mechanisms, such as disk drives, that are incorporated in most computers. Here, there are extended range computer communications links that allow various degrees of separation between the computer and monitor and keyboard, as set forth in U.S. Pat. No. 5,257,390, issued to Asprey, and which is incorporated herein by reference. Additionally, situations occur wherein it may be desirable to locate a computer coupled to a color monitor in a control room and to locate an inexpensive IBM standard monochrome monitor, also coupled to the computer by one of the aforementioned extended links, in a factory or manufacturing environment, or other inhospitable environment, a distance away from the computer.
These inexpensive, IBM standard monitors are driven by an analog monochrome video signal applied to the G, or green, video signal output terminal of a 15-terminal subminiature high density "D" connector mounted on the computer, also as specified by IBM VGA standards. Particularly, in PC-type computers outputting VGA color video signals, the color signals are provided from the computer as discrete R (red), G (green), and B (blue) video signals. These color signals are provided on terminals occupying specified positions in the 15-terminal connector, the terminals being designated R, G, and B and the connector couplable also to a color VGA monitor. As stated, in the monochrome mode, a single video signal is applied to the terminal occupying the G position, which is coupled as an input to the monochrome monitor. In the instance of a monochrome monitor being coupled to the computer, the monochrome video signal drives the monitor image at only four levels of brightness or intensity, these levels being OFF, DIM, NORMAL, and BRIGHT, with the most intense level, BRIGHT, being developed by a signal of about 700 millivolts. The NORMAL and DIM levels of intensity are developed by progressively smaller amplitudes of the monochrome video signal.
Where software for a color monitor provides color backgrounds for highlighting and a different color of data on the highlighted backgrounds, problems arise when the signal for a color monitor is split and one of the color signals fed to the monochrome monitor or the color signal itself is applied to a monochrome monitor. Here, since the color signals are provided on R, G, and B terminals, and the monochrome monitor is only provided with a G terminal input for both the background and data, any red and blue data on the R and B terminals is not displayable on the monitor. Further, where there are mixtures of colors containing a green component in the color background, such as yellow or cyan, and data containing a green component of equal intensity as the green component of the background is superimposed on the background, the green component in the background colors masks the green component of the data. For example, yellow characters on a cyan background, when applied to a monochrome IBM standard monitor, produces an all-white display, with the characters being indistinguishable from the background.
This particular problem has been in existence for several years, with attempts to overcome the problem being software oriented. These solutions utilize various symbols to denote colors in a monochrome environment and do not represent color data in a manner that is pleasing to the eye or readily discernible as a contrast, such as the aforementioned colored background and differently colored letters.
One patent of which applicant is aware is U.S. Pat. No. 4,641,262 issued to Bryan. In this patent, TTL level CGA red, green, blue, and intensity signals are converted to a maximum of 16 shades of gray. Further, the Bryan system may be properly labelled one wherein the color signals are algebraically proportionally summed, meaning that where more than one color signal is present, a higher signal adds to the resultant signal value, and a lower signal subtracts from the resultant signal value. In it, the TTL levels, generally designated as either high or low, range in value, with high signals being on the order of 3 to 5 volts and low signals being on the order of 0 to 2 volts. The effect of binary weighting multiplies each signal by 1, 2, 4, or 8. Perhaps the most significant effect that can arise with this system is that where there is a high value color signal, thus indicating a significant brightness for that signal, and there is a lesser value or 0 from another, the result will be that the overall brightness is inordinately reduced. This results from the straight proportional summing of signal values at a summing junction.
In any event, the applicant finds that such an algebraic combination of signals, or summing, does not provide for an ideal combination of color signals for a monochrome presentation.
Also, there is no provision in Bryan to operate more than one monitor simultaneously or to easily configure the hardware to provide both color or monochrome signals to, for example, two monitors or to locate either of the monitors at a remote distance from the computer.
Accordingly, it is an object of this invention to provide simple, reliable circuitry that receives analog VGA color video signals from a computer and converts, but does not algebraically add, these signals to monochrome signals for an IBM standard analog VGA monochrome monitor wherein the conversion results in an improved gray scale.